Community antenna television (CATV) signals used for cable television transmission may be transmitted over optical communication networks, using intensity-modulated analog lightwave signals. Lasers are typically employed to generate the light signals. A CATV signal generally consists of several carrier signals representing different television channels. The analog lightwave systems used for transmitting these CATV signals have stringent carrier-to-noise ratio (CNR) and distortion requirements.
A source of noise referred to as interferometric intensity noise (IIN) can degrade the CNR, thereby compromising system performance. IIN is caused predominantly by multipath interference (MPI), which is interference that results from multiple reflections of the lightwave signal in optical fiber.
In particular, MPI is detrimental to analog lightwave systems because it converts source phase noise to lightwave intensity noise, which increases the system noise level. For a further discussion of MPI, see Judy, "Intensity Noise from Fiber Rayleigh Backscatter and Mechanical Splices," Proc. 15th European Conf. Optical Communications (Gothenburg, Sweden, Sep. 10, 1989). The spectrum of the noise caused by IIN is strongly dependent on the optical spectrum of the modulated laser. Lasers exhibiting relatively broad linewidths produce phase noise that falls within the CATV signal band. Consequently, IIN in systems employing such lasers can degrade signal quality.
Interference noise may also be caused by single reflections of bidirectional light transmission, referred to herein as single reflection noise, which may occur in interactive CATV applications. In bidirectional systems, light travels in both directions along the optical fiber. If light originating at one end of the system is reflected, then the reflected light may interfere with the light originating from the other end of the system. Because of this interference, the optical frequency differences between the light from the two sources are converted to intensity noise. When the light sources' optical frequencies are close this noise may fall within the frequency band of interest, thereby degrading system performance.
MPI noise and single reflection noise within the CATV band may be reduced by distributing the total noise spectrum over a broader frequency range. It is known that by increasing the spectral width of the laser used to transmit the optical signals, the noise concentration due to IIN may be spread over a wide range of frequencies. See Judy, supra. Consequently, broadening the optical spectrum of the transmitting laser reduces the concentration of IIN at the frequencies within the CATV signal band.
In the past, transmission systems using direct intensity-modulated semiconductor lasers relied on the FM efficiency of the lasers, also known as chirp, to broaden the optical spectrum. Darcie, et al, "Fiber-Reflection-Induced Impairments in Lightwave AM-VSB CATV Systems," 9 J. Lightwave Tech. 991 (1991). Chirp is known in the art as the incidental modulation of the light's wavelength or frequency that occurs during direct intensity modulation of a light source. Chirp resulting from direct modulation by an amplitude modulated-vestigial sideband format (AM-VSB) CATV signal can broaden the optical spectral width, thereby reducing the noise caused by MPI.
Not all of the effects of chirp, however, are beneficial. Detrimental effects caused by the interaction of chirp and polarization-mode dispersion (PMD) or chromatic dispersion in the fiber increase as the frequency of the modulation increases. CATV signals are carried at frequencies up to 500 MHz or more. As a result, the use of chirp produced by CATV signals can cause an unacceptable level of signal degradation due to chirp-related impairments. See Phillips, et al, "Non-linear Distortion Generated by Dispersive Transmission of Chirped Intensity-Modulated Signals," 3 IEEE Photonic Tech. Letters 481-83 (1991). This forces the system designer to strike a delicate balance between too much chirp, where dispersion creates problems, and not enough, where multipath interference limits system performance.
An alternative proposal to broaden the optical spectrum is to employ a fast phase modulator in the optical transmission system. See Yariv, et al., "A Reduction of Interferometric Phase-to-Intensity Conversion Noise in Fiber Links by Large Index Phase Modulation of the Optical Beam," 10 J. Lightwave Tech. 978-981 (July 1992). Although this method can lower the noise due to MPI, this method has the disadvantage of requiring the incorporation of a fast phase modulator, which significantly increases the cost of the system.